The present invention relates to a method of expanding types of synchronous motors to obtain synchronous motors having different specification values with respect to torque and/or inertia, and synchronous motors produced by the method.
As an alternating-current motor, there is known a synchronous motor comprising a rotor having permanent magnets therein and a stator having windings thereon. In setting types of synchronous motors, motor characteristics of torque and rotor inertia are determined to have different specification values in series of motors, to thereby obtain expanded types of synchronous motors.
There are defined a variety of series such as a standard series having standard specification values of torque and rotor inertia, a low-inertia series having a small rotor inertia specification value relative to the torque specification value, and a high-torque series having a large torque specification value relative to the rotor inertia specification value, and motors in each series have output torque and rotor inertia values different from one another to be approximate multiples of that of the other one.
FIGS. 4a and 4b illustrate the arrangements of conventional synchronous motors, wherein FIG. 4a shows the combination of a stator and a rotor for constructing a standard-series motor having standard specification values of torque and rotor inertia, and FIG. 4b shows the combination of a stator and a rotor for constructing a low-inertia series motor having a smaller rotor inertia specification value.
In FIG. 4a, a standard series synchronous motor 10 is constituted by combination of a stator 11 and a rotor 12. The stator 11 comprises a stack of steel plates with an overall height H, each steel plate having a center hole 15 for receiving the rotor 12 therein and grooves 13 for fitting windings on an inner periphery of the hole 15. The rotor 12 has a diameter h1, such that it can be inserted into the hole 15 of the stator 11, and has permanent magnets 14 arranged circumferentially.
A low-inertia series synchronous motor 20 shown in FIG. 4b is constituted by combination of a stator 21 and a rotor 22. The stator 21 comprises a stack of steel plates with the overall height H, each steel plate having a center hole 25 for receiving the rotor 22 therein and grooves 23 for fitting windings on the inner periphery of the hole 25. The rotor 22 has a diameter h2 such that it can be inserted into the hole 25 of the stator 21, and has permanent magnets 24 arranged circumferentially. To reduce the rotor inertia, the diameter h2 of the rotor 22 is smaller than the diameter h1 of the rotor 12 for the standard series, and the stator 21 also has a smaller inner diameter corresponding to the small diameter h2 of the rotor 22. Generally, the permanent magnets 14 and 24 used in the rotors of the conventional synchronous motors are made of a magnetic material such as ferrite.
Thus, the conventional synchronous motor has a construction such that one stator shape is associated with one type of rotor to be inserted into the stator, and the synchronous motor characteristics and the stator shape are in one-to-one relation. Each series of synchronous motors is therefore constructed by the selective combination of one of groups of stators having an identical sectional shape but different stack lengths, with rotors associated with the selected stator group.
FIGS. 5 and 6 illustrate a conventional method for expanding types of synchronous motor, wherein FIG. 5 shows stator groups and rotor groups in the conventional method, and FIG. 6 shows standard-series motors and low-inertia series motors constructed by combining the stator groups and the rotor groups.
In FIG. 5, the stator groups and the rotor groups are shown on the left-hand and right-hand sides of the figure, respectively. The stator groups consist of a large-diameter stator group including stators SA, SB, SC and SD having a large-diameter hole for receiving a rotor therein, and a small-diameter stator group including stators Sa, Sb and Sc having a small-diameter hole for receiving a rotor therein. For the rotor groups, two different outer diameters, that is, large and small outer diameters, are set in accordance with to the rotor inertia, and thus the rotor groups consist of a large-diameter rotor group including rotors RA, RB, RC and RD having large diameters, and a small-diameter rotor group including rotors Ra, Rb and Rc having small diameters. The stators and the rotors have their diameters and lengths set in accordance with characteristics of synchronous motors to be obtained.
The heights of the stators SA-SD and the rotors RA-RD are set to be multiples of L, i.e., L, 2L, 4L, 8L, . . . and the heights of the stators Sa-Sc and the rotors Ra-Rc are set to be multiples of L, i.e., L, 2L, 4L, . . . .
To construct a plurality of series of synchronous motors using stator groups having respective identical sectional shapes and different stack lengths, stator-rotor combinations are selected from among the stator and rotor groups shown in FIG. 5 in accordance with required synchronous motor characteristics, thereby obtaining synchronous motors of standard series and low-inertia series as shown in FIG. 6. In FIG. 6, the right-hand side shows standard series motors having standard torque and rotor inertia specification values, and the left-hand side shows low-inertia series motors having relatively small rotor inertia specification values. Each series aligned in a column comprises stator-rotor combinations of which the torque values and inertia values are respectively different from one another to be multiples of a fundamental value.
For example, in a first row across the two series (the uppermost horizontal combination), a standard series motor has a rotor inertia specification value of J and a torque specification value of T. To construct a low-inertia series motor having a smaller rotor inertia specification value, a stator and a rotor both having a smaller diameter are selected and combined, whereby an expanded type of synchronous motor is obtained.
In the conventional method of expanding synchronous motor types, one stator shape is associated with one type of rotor to be inserted in the stator and the synchronous motor characteristics and the stator shape are in one-to-one relation, as stated above. Accordingly, to construct a series of synchronous motors using a group of stators having the same sectional shape but different stack lengths, as many stator types as the rotor types are required, giving rise to a problem that a large number of types of stators are needed.
The conventional method for expanding synchronous motor types also has a problem that when the stator height and the rotor length are changed in order to alter the torque specification value, the rotor inertia also changes with the change of the stator height and the rotor length.
Table 1 above shows how respective torque and rotor inertia specification values are achieved by the standard series motors and the low-inertia series motors according to the conventional synchronous motor type expansion method. For example, a motor with the torque specification value T and the rotor inertia specification value J can be realized by a standard series type A, and a motor with the torque specification value T and the rotor inertia specification value J/2 can be realized by a low-inertia series type a. In Table 1, symbols A to C and a to c affixed to the end of the respective series names denote motor types of which the stator-rotor combinations are shown in FIG. 6.
Referring to Table 1, in the case where the torque specification value of the standard series type A is required to be changed to 2T, such requirement can be satisfied by the standard series type B in the conventional synchronous motor type expansion method, as indicated by the broken-line arrow. However, in this case, not only the torque but the rotor inertia increases doubly, and it is not possible to change only the torque specification value without changing the rotor inertia.
An object of the present invention is to reduce the number of stator types needed and thereby permit stators to be used in common in cases where a plurality of series of motors are constructed by using a group of stators having an identical sectional shape and different stack lengths. Another object of the invention is to cut down the cost of expanding synchronous motor types by the common use of stators.
Still another object of the present invention is to permit expansion of synchronous motor types in such a manner that only the torque specification value or only the rotor inertia specification value can be changed.
A synchronous motor type expansion method of the present invention comprises the step of: preparing a group of stators by stacking stator cores of identical shape so that heights of the stators are different from one another to be multiples of a fundamental height in accordance with predetermined output torque specification values and predetermined rotor inertia specification values; preparing a plurality of groups of rotors so that lengths of the rotors in each group are different from one another to be multiples of a fundamental length in accordance with the predetermined rotor inertia specification values, the rotors in each group being provided with permanent magnets having a residual magnetic flux density different from that of permanent magnets of the rotors in another group in accordance with the preset output torque specification values; selecting a stator from the prepared group of stators and a rotor from the prepared groups of rotors in accordance with a preset output torque specification value and a preset rotor inertia specification value and combining the selected stator with the selected rotor.
According to the motor type expansion method of the present invention, a stator and a rotor are selected in accordance with desired torque and rotor inertia for a synchronous motor to be constituted from the group of stators and the groups of rotors prepared in advance, and are combined with each other. Each stator in the stator group can be formed by a single type of stator cores having an identical shape to have different height, so that the number of stator types is reduced, permitting the stators to use in common.
Various stators in the stator group necessary for expanding the motor type can be formed by changing only the number of stator cores of identical shape to be stacked, and thus the stator cores can be used in common. Various rotors in the rotor groups necessary for expanding the motor type can be prepared by a combination of different residual magnetic flux densities of permanent magnets and different rotor lengths.
For changing the rotor inertia value according to the motor type expansion method of the present invention, the residual magnetic flux density of permanent magnets and the rotor length are altered to thereby change the rotor inertia value without changing the torque value. On the other hand, for changing the torque value, the residual magnetic flux density of permanent magnets is altered to thereby change the torque value without changing the rotor inertia value.
According to the motor type expansion method of the present invention, a plurality of series of synchronous motors are produced using the group of stators having an identical shape and different stack lengths to permit the stators to be used in common, thus making it possible to reduce the number of stator types to produce synchronous motors at low cost.
The permanent magnets are arranged radially with respect to the rotor. In this case, the permanent magnets are attached to the rotor in various ways; for example, they may be affixed to the surface of the rotor or be embedded in the core of the rotor.